Before this patch, chg always uses color in its debugmsg and abortmsg and
there is no way to turn it off.
This patch adds a global flag to control whether chg should use color or
not and only enables it when stderr is a tty and HGPLAIN is not set.
Before this patch, if __GNUC__ is not defined, PRINTF_FORMAT_ will not be
defined and will cause compilation error.
This patch solves the issue by making sure PRINTF_FORMAT_ is defined. It
allows chg to be compiled with tcc (http://bellard.org/tcc/) by:
tcc -o chg *.c
They are like malloc and realloc but will abort the program on error.
A lot of places use {m,re}alloc and check their results. This patch
can simplify them.
